For many, many years, humans have sought ways to correct visual problems. The ancient Chinese slept with small bags of mercury on their eyes, flattening their corneas and improving their shortsightedness. Unfortunately, the effects only worked for a few minutes after waking. Spectacles are thought to have been first introduced by the Arabs in the 11th Century and were introduced into Europe about 200 years later. This century has seen the development of contact lenses, initially the hard variety and later soft and disposable soft lenses.
Although these optical aids allow patients to see well while wearing them, they do not offer a permanent cure for the visual disorder or problem, and in some situations even glasses and contacts cannot provide complete correction due, for example, to a localized, highly irregular shaped corneal defect. Also, in many situations, they are inappropriate, for example, when swimming or wearing contacts in the laboratory. Another problem is that in some instances dangerous situations can arise when they become dislodged. This can occur while they are being used by firefighters and police officers.
Roughly two decades ago, surgical techniques were introduced in an effort to permanently correct shortsightedness and astigmatism. The radial keratotomy procedure used a diamond blade to make incisions into the cornea, the front surface or "window of the eye". Although this technique worked relatively well, there have been problems with long term stability of vision and weakening of the cornea as a result of the cuts often having to be made up to 95% of the corneal thickness.
More recently, these older techniques have been replaced with laser treatment techniques which have replaced the surgeon's blade with a computer controlled laser that gently re-sculptures the shape of the cornea without cutting or, for most applications, weakening the eye. These laser techniques are typically carried out with a photoablation process using an excimer laser.
Excimer lasers were chiefly developed for the manufacture of computer microchips, where they were used to etch the circuits. However, the laser's extreme accuracy and low thermal effect resulted in it being well suited as an eye laser. That is, many eye lasers are extremely accurate and remove only 0.25 microns (1/4000.sup.th millimeter) of tissue per pulse. During the re-sculpturing, the excimer laser gently "evaporates" or vaporizes tissue; there is no burning or cutting involved. In most cases, the laser treatment takes only 20 to 45 seconds, depending on how severe the refractive error is. A fast treatment time is important in that, for some procedures, an overextended treatment period can slow the post operative curing process to final vision level obtainment.
In the normal eye, light rays entering the eye are accurately focused on the retina and a clear image is formed. Most of the bending or focusing of the light rays occurs at the cornea, with the natural lens inside the eye being responsible for fine adjustments. If light is not focused on the retina, then the eye is said to have a refractive error. Common refractive errors include: myopia or shortsightedness, hyperopia or farsightedness, and astigmatism. The excimer laser has been used to re-sculpture the cornea in myopia, hyperopia and astigmatism corrections in an effort to make the curve of the cornea focus light rays normally on the retina.
Myopia, or shortsightedness, is a condition whereby light rays come to a focus in front of, rather than on, the retina at the back of the eye. This results in blurry vision, especially when looking at objects far away. Myopia results from the length of the eye being too long or the cornea being too steeply curved.
In hyperopia, or farsightedness, light rays are focused behind the retina. This results in blurry vision especially when looking at objects that are close. Hyperopia results from the length of the eye being too short or the cornea being too flat.
In astigmatism, the cornea, or window of the eye, has an irregular curvature being shaped more like a rugby ball, rather than a soccer ball. Light rays are focused at different points. A person often has some degree of astigmatism and myopia or hyperopia at the same time. Any surface contour irregularities can also result in the improper focusing of the eye due to the irregularitics causing light rays to land away from the desired focal point on the retina.
In myopia laser correction procedures, the cornea is flattened to better focus light rays normally on the retina, whereas in hyperopia, the cornea is made more curved. With astigmatism, the surface of the cornea is re-sculptured to a regular curvature.
Presbyopia is a problem considered to be due to an aging process occurring in the natural lens of the eye, and thus does not fall under the same category as the refractive errors of myopia, astigmatism and hyperopia noted above, although combinations of presbyopia and one or more of the refractive errors are possible. US Pat. No. 5,533,997 to Dr. Luis A. Ruiz describes a presbyopia corrective apparatus and method which involves the use of a laser system to remove tissue from the eye in presbyopic corrective patterns discovered to be effective by the inventor.
One of the prior art laser treatment methods is known as photorefractive keratectomy (PRK), in which the laser beam is applied directly to the surface of the cornea, after the thin surface layer of epithelium cells has been removed (e.g., through solvent with wiping, preliminary laser treatment, or minor abrasion). After the direct laser re-sculpturing of the cornea, a bare area of the cornea is left which takes a few days to heal (e.g., 2 to 6 days) and can be uncomfortable during this period. The healing process can sometimes lead to regression (some refractive error returns) or to scanning (which may blur the vision), especially in patients with large refractive errors. Although still used for low degrees of myopia and hyperopia, PRK is generally being replaced by the LASIK method for these same disorders, in which the laser treatment is applied under a protective corneal flap. Under the "Laser in situ Keratomileusis" (LASIK) treatment, a thin protective corneal flap is raised, rather like a trapdoor. The front surface of the exposed cornea is treated by the excimer laser.
The net result being that the cornea is altered in a manner directed at allowing light rays to be focused normally on the retina. At the end of the procedure, the protective flap is simply replaced. The LASIK technique leaves the original surface of the cornea virtually intact, hence, there is no bare area to cause pain. In addition, the mild healing process results in minimal regression and avoids scarring problems.
The ablation profiles for the prior art PRK and LASIK laser treatments described above are based on mathematical equations and formulas that assume the eye as a perfect optical body or one that conforms to an optical model having very regular spherical shapes. The prior art ablation profiles thus fail to take into consideration the fact that each eye is unique and possesses many individual and general small and large irregularities. Because the prior art ablation profiles are based on fixed and regular ablation patterns, there can be created situations where excessive tissue is removed or insufficient tissue is removed. For example, in certain astigmatism situations there is a much larger defect on one side as compared to the opposite diametrical side. Thus, upon application of a normal, prior art laser ablation pattern for such a situation (an eliptical ablation profile), the ablation pattern would remove both the tissue causing the defect and tissue not associated with the defect, thus creating the possibility of a new defect in the eye following treatment.
Also, the corneal surface is not a very smooth body and has topographical irregularities which can be both large and small. Under the prior art laser systems these surface irregularities are not taken into consideration in the formulas and patterns designed to correct defects such as hyperopia, myopia and astigmatism. Accordingly, the final ablation profile formed in the eye will deviate to some extent from what was predetermined by the surgeon to be the final resultant profile of the eye, and this is particularly true with respect to eyes with highly irregular surfaces wherein the defect can be simply shifted to a lower corneal altitude and thus create a new defect which is often unpredictable under the prior art systems. This would be true for both PRK and LASIK treatments as in the former the laser would ablate deeper into the eye then what was originally contemplated in any valley area in the topography of the eye and not as deep as expected in any peak or protrusion area of the topography. With LASIK, the microkeratome is designed to remove a constant thickness flap by way of pressing down during the cutting or planarization process such that the topography of the external surface of the cornea is duplicated in the exposed corneal stroma therebelow.
Because the prior art systems rely on rigid patterns and formulas that are based on standard optical models, they limit the surgeon from fully exercising his clinical expertise during the determination of an ablation profile to be performed. In other words, they do not allow a surgeon to customize an ablation profile to best suit the surgeon's clinical evaluation of the patients corrective requirements.
The prior art systems are also not well suited for many eye corrections that require fine detail or customized ablations particularly eye correction cases such as trauma, some congenital defects, and defects that arise due to accidents during eye surgery.
The following articles, patents and patent application provide additional background information and are incorporated herein by reference:
U.S. Pat. Nos. 4,721,370 (L'Esperance); 4,995,716 (Wamicki et al); 5,133,726 (Ruiz et al.); 5,159,361 (Cambier et al) 5,318,046(Rozakis); 5,533,997 (Ruiz et al.) and 5,843,070 (Cambier et al) and pending U.S. patent application Ser. No. 09/186,884 to Luis A. Ruiz.
"Corneal Topography--The state of the Art" James P. Gill et. al. Published by Slack Incorporated. PA0 Ren, Qiushi, Richard H. Keates, Richard A. Hill, and Michael W. Berns. "Laser Refractive Surgery: A Review and Current Status." Optical Engineering, 34, 642-59 (1995). PA0 Lin, J. T. "Critical Review on Refractive Surgical Lasers." Optical Engineering, 34, 668-75 (1995). PA0 Munnerlyn, Charles R., Stephen J. Koons and John Marshall. "Photorefractive Keratectomy: A Technique for Laser Refractive Surgery." J. Cataract Refract. Surg. 14, 46-52 (January 1988). PA0 Manns, Fabrice, Jui-Hui Shen, Per Soderberg, Takaaki Matsui, and Jean-Marie Parel. "Development of an Algorithm for Corneal Reshaping With a Scanning Laser Beam." Applied Optics, 34, 4600-08 (July 1995).
Chapter 3. "Characterizing Astigmatism: Keratometric Measurements Do Not Always Accurately Reflect Conical Topography." 25-33. PA1 Chapter 5. Thornton, Spencer P. and Joseph Wakil. "The EyeSys 2000 Corneal Analysis System." 55-75. PA1 Chapter 7. Snook, Richard K. "Pachymetry and True Topography Using the ORBSCAN System." 89-103. PA1 Chapter 9. Smolek, Michael K. and Stephen D. Klyce. "The Tomey Technology/Computed Anatomy TMS-1 Videokeratoscope." 123-48. PA1 Chapter 16. Durrie, Daniel S., Donald R. Sanders, D. James Schumer, Manus C. Kraff, Robert T. Spector, and David Gubman. "Evaluating Excimer Laser Procedures." 141-61.